docs: def _null_date(self):
          """
          Returns date which is represented by a integer 0.
          """
          # http://www.openoffice.org/api/docs/common/ref/com/sun/star/util/NumberFormatSettings.html#NullDate
          try:
              return self.__null_date
          except AttributeError:
              number_settings = self._target.getNumberFormatSettings()
              d = number_settings.getPropertyValue('NullDate')
              self.__null_date = datetime.datetime(d.Year, d.Month, d.Day)
          return self.__null_date